Add pgp-wrapper example

Creates an OpenPGP certificate based on the SSH key and allows signing files
emitting OpenPGP framed packets.

Requires that the first key in SSH is ed25519 (see `ssh-add -L`).

Generate a key with:
`cargo run --example pgp-wrapper generate "John Doe <john@example.com>" > key.pgp`

Sign data using:
`cargo run --example pgp-wrapper sign < Cargo.toml > Cargo.toml.sig`

Import the certificate using GnuPG:
```sh
$ gpg --import key.pgp
gpg: key A142E92C91BE3AD5: public key "John Doe <john@example.com>" imported
gpg: Total number processed: 1
gpg:               imported: 1
```

Verify the signature using GnuPG:
```sh
$ gpg --verify Cargo.toml.sig
gpg: assuming signed data in 'Cargo.toml'
gpg: Signature made Fri May 10 11:15:53 2024 CEST
gpg:                using EDDSA key 4EB27E153DDC454364B36B59A142E92C91BE3AD5
gpg: Good signature from "John Doe <john@example.com>" [unknown]
gpg: WARNING: This key is not certified with a trusted signature!
gpg:          There is no indication that the signature belongs to the owner.
Primary key fingerprint: 4EB2 7E15 3DDC 4543 64B3  6B59 A142 E92C 91BE 3AD5
```

Works perfectly in conjunction with `openpgp-card-agent.rs`!

Signed-off-by: Wiktor Kwapisiewicz <wiktor@metacode.biz>

Author: wiktor-k